RFC 3310 HTTP Digest Authentication Using AKA September 2002
This document specifies a mapping of AKA parameters onto HTTP Digest
authentication. In essence, this mapping enables the usage of AKA as
a one-time password generation mechanism for Digest authentication.
As the Session Initiation Protocol (SIP) [3] Authentication Framework
closely follows the HTTP Authentication Framework, Digest AKA is
directly applicable to SIP as well as any other embodiment of HTTP
Digest.
1.1 Terminology
This chapter explains the terminology used in this document.
AKA
Authentication and Key Agreement.
AuC
Authentication Center. The network element in mobile networks
that can authorize users either in GSM or in UMTS networks.
AUTN
Authentication Token. A 128 bit value generated by the AuC, which
together with the RAND parameter authenticates the server to the
client.
AUTS
Authentication Token. A 112 bit value generated by the client
upon experiencing an SQN synchronization failure.
CK
Cipher Key. An AKA session key for encryption.
IK
Integrity Key. An AKA session key for integrity check.
ISIM
IP Multimedia Services Identity Module.
PIN
Personal Identification Number. Commonly assigned passcodes for
use with automatic cash machines, smart cards, etc.
RAND
Random Challenge. Generated by the AuC using the SQN.
RES
Authentication Response. Generated by the ISIM.
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
SIM
Subscriber Identity Module. GSM counter part for ISIM.
SQN
Sequence Number. Both AuC and ISIM maintain the value of the SQN.
UMTS
Universal Mobile Telecommunications System.
XRES
Expected Authentication Response. In a successful authentication
this is equal to RES.
1.2 Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [1].
2. AKA Mechanism Overview
This chapter describes the AKA operation in detail:
1. A shared secret K is established beforehand between the ISIM and
the Authentication Center (AuC). The secret is stored in the
ISIM, which resides on a smart card like, tamper resistant device.
2. The AuC of the home network produces an authentication vector AV,
based on the shared secret K and a sequence number SQN. The
authentication vector contains a random challenge RAND, network
authentication token AUTN, expected authentication result XRES, a
session key for integrity check IK, and a session key for
encryption CK.
3. The authentication vector is downloaded to a server. Optionally,
the server can also download a batch of AVs, containing more than
one authentication vector.
4. The server creates an authentication request, which contains the
random challenge RAND, and the network authenticator token AUTN.
5. The authentication request is delivered to the client.
6. Using the shared secret K and the sequence number SQN, the client
verifies the AUTN with the ISIM. If the verification is
successful, the network has been authenticated. The client then
produces an authentication response RES, using the shared secret K
and the random challenge RAND.
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
7. The authentication response, RES, is delivered to the server.
8. The server compares the authentication response RES with the
expected response, XRES. If the two match, the user has been
successfully authenticated, and the session keys, IK and CK, can
be used for protecting further communications between the client
and the server.
When verifying the AUTN, the client may detect that the sequence
numbers between the client and the server have fallen out of sync.
In this case, the client produces a synchronization parameter AUTS,
using the shared secret K and the client sequence number SQN. The
AUTS parameter is delivered to the network in the authentication
response, and the authentication can be tried again based on
authentication vectors generated with the synchronized sequence
number.
For a specification of the AKA mechanism and the generation of the
cryptographic parameters AUTN, RES, IK, CK, and AUTS, see reference
3GPP TS 33.102 [6].
3. Specification of Digest AKA
In general, the Digest AKA operation is identical to the Digest
operation in RFC 2617 [2]. This chapter specifies the parts in which
Digest AKA extends the Digest operation. The notation used in the
Augmented BNF definitions for the new and modified syntax elements in
this section is as used in SIP [3], and any elements not defined in
this section are as defined in SIP and the documents to which it
refers.
3.1 Algorithm Directive
In order to direct the client into using AKA for authentication
instead of the standard password system, the RFC 2617 defined
algorithm directive is overloaded in Digest AKA:
algorithm = "algorithm" EQUAL ( aka-namespace
/ algorithm-value )
aka-namespace = aka-version "-" algorithm-value
aka-version = "AKAv" 1*DIGIT
algorithm-value = ( "MD5" / "MD5-sess" / token )
algorithm
A string indicating the algorithm used in producing the digest and
the checksum. If the directive is not understood, the nonce
SHOULD be ignored, and another challenge (if one is present)
should be used instead. The default aka-version is "AKAv1".
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
Further AKA versions can be specified, with version numbers
assigned by IANA [7]. When the algorithm directive is not
present, it is assumed to be "MD5". This indicates, that AKA is
not used to produce the Digest password.
Example:
algorithm=AKAv1-MD5
If the entropy of the used RES value is limited (e.g., only 32
bits), reuse of the same RES value in authenticating subsequent
requests and responses is NOT RECOMMENDED. Such a RES value
SHOULD only be used as a one-time password, and algorithms such as
"MD5-sess", which limit the amount of material hashed with a
single key, by producing a session key for authentication, SHOULD
NOT be used.
3.2 Creating a Challenge
In order to deliver the AKA authentication challenge to the client in
Digest AKA, the nonce directive defined in RFC 2617 is extended:
nonce = "nonce" EQUAL ( aka-nonce
/ nonce-value )
aka-nonce = LDQUOT aka-nonce-value RDQUOT
aka-nonce-value = <base64 encoding of RAND, AUTN, and
server specific data>
nonce
A parameter, which is populated with the Base64 [4] encoding of
the concatenation of the AKA authentication challenge RAND, the
AKA AUTN token, and optionally some server specific data, as in
Figure 1.
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
Example:
nonce="MzQ0a2xrbGtmbGtsZm9wb2tsc2tqaHJzZXNy9uQyMzMzMzQK="
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| RAND |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| AUTN |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server Data...
+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Generating the nonce value.
If the server receives a client authentication containing the "auts"
parameter defined in Section 3.4, that includes a valid AKA AUTS
parameter, the server MUST use it to generate a new challenge to the
client. Note that when the AUTS is present, the included "response"
parameter is calculated using an empty password (password of ""),
instead of a RES.
3.3 Client Authentication
When a client receives a Digest AKA authentication challenge, it
extracts the RAND and AUTN from the "nonce" parameter, and assesses
the AUTN token provided by the server. If the client successfully
authenticates the server with the AUTN, and determines that the SQN
used in generating the challenge is within expected range, the AKA
algorithms are run with the RAND challenge and shared secret K.
The resulting AKA RES parameter is treated as a "password" when
calculating the response directive of RFC 2617.
3.4 Synchronization Failure
For indicating an AKA sequence number synchronization failure, and to
re-synchronize the SQN in the AuC using the AUTS token, a new
directive is defined for the "digest-response" of the "Authorization"
request header defined in RFC 2617:
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
auts = "auts" EQUAL auts-param
auts-param = LDQUOT auts-value RDQUOT
auts-value = <base64 encoding of AUTS>
auts
A string carrying a base64 encoded AKA AUTS parameter. This
directive is used to re-synchronize the server side SQN. If the
directive is present, the client doesn't use any password when
calculating its credentials. Instead, the client MUST calculate
its credentials using an empty password (password of "").
Example:
auts="CjkyMzRfOiwg5CfkJ2UK="
Upon receiving the "auts" parameter, the server will check the
validity of the parameter value using the shared secret K. A valid
AUTS parameter is used to re-synchronize the SQN in the AuC. The
synchronized SQN is then used to generate a fresh authentication
vector AV, with which the client is then re-challenged.
3.5 Server Authentication
Even though AKA provides inherent mutual authentication with the AKA
AUTN token, mutual authentication mechanisms provided by Digest may
still be useful in order to provide message integrity.
In Digest AKA, the server uses the AKA XRES parameter as "password"
when calculating the "response-auth" of the "Authentication-Info"
header defined in RFC 2617.
4. Example Digest AKA Operation
Figure 2 below describes a message flow describing a Digest AKA
process of authenticating a SIP request, namely the SIP REGISTER
request.
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RFC 3310 HTTP Digest Authentication Using AKA September 20025.1 Authentication of Clients using Digest AKA
AKA is typically -- though this isn't a theoretical limitation -- run
on an ISIM application that usually resides in a tamper resistant
smart card. Interfaces to the ISIM exist, which enable the host
device to request authentication to be performed on the card.
However, these interfaces do not allow access to the long-term secret
outside the ISIM, and the authentication can only be performed if the
device accessing the ISIM has knowledge of a PIN code, shared between
the user and the ISIM. Such PIN codes are typically obtained from
user input, and are usually required when the device is powered on.
The use of tamper resistant cards with secure interfaces implies that
Digest AKA is typically more secure than regular Digest
implementations, as neither possession of the host device nor Trojan
Horses in the software give access to the long term secret. Where a
PIN scheme is used, the user is also authenticated when the device is
powered on. However, there may be a difference in the resulting
security of Digest AKA, compared to traditional Digest
implementations, depending of course on whether those implementations
cache/store passwords that are received from the user.
5.2 Limited Use of Nonce Values
The Digest scheme uses server-specified nonce values to seed the
generation of the request-digest value. The server is free to
construct the nonce in such a way, that it may only be used from a
particular client, for a particular resource, for a limited period of
time or number of uses, or any other restrictions. Doing so
strengthens the protection provided against, for example, replay
attacks.
Digest AKA limits the applicability of a nonce value to a particular
ISIM. Typically, the ISIM is accessible only to one client device at
a time. However, the nonce values are strong and secure even though
limited to a particular ISIM. Additionally, this requires that the
server is provided with the client identity before an authentication
challenge can be generated. If a client identity is not available,
an additional round trip is needed to acquire it. Such a case is
analogous to an AKA synchronization failure.
A server may allow each nonce value to be used only once by sending a
next-nonce directive in the Authentication-Info header field of every
response. However, this may cause a synchronization failure, and
consequently some additional round trips in AKA, if the same SQN
space is also used for other access schemes at the same time.
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RFC 3310 HTTP Digest Authentication Using AKA September 20025.3 Multiple Authentication Schemes and Algorithms
In HTTP authentication, a user agent MUST choose the strongest
authentication scheme it understands and request credentials from the
user, based upon that challenge.
In general, using passwords generated by Digest AKA with other HTTP
authentication schemes is not recommended even though the realm
values or protection domains would coincide. In these cases, a
password should be requested from the end-user instead. Digest AKA
passwords MUST NOT be re-used with such HTTP authentication schemes,
which send the password in clear. In particular, AKA passwords MUST
NOT be re-used with HTTP Basic.
The same principle must be applied within a scheme if several
algorithms are supported. A client receiving an HTTP Digest
challenge with several available algorithms MUST choose the strongest
algorithm it understands. For example, Digest with "AKAv1-MD5" would
be stronger than Digest with "MD5".
5.4 Online Dictionary Attacks
Since user-selected passwords are typically quite simple, it has been
proposed that servers should not accept passwords for HTTP Digest,
which are in the dictionary [2]. This potential threat does not
exist in HTTP Digest AKA because the algorithm will use ISIM
originated passwords. However, the end-user must still be careful
with PIN codes. Even though HTTP Digest AKA password requests are
never displayed to the end-user, she will be authenticated to the
ISIM via a PIN code. Commonly known initial PIN codes are typically
installed to the ISIM during manufacturing and if the end-users do
not change them, there is a danger that an unauthorized user may be
able to use the device. Naturally this requires that the
unauthorized user has access to the physical device, and that the
end-user has not changed the initial PIN code. For this reason,
end-users are strongly encouraged to change their PIN codes when they
receive an ISIM.
5.5 Session Protection
Digest AKA is able to generate additional session keys for integrity
(IK) and confidentiality (CK) protection. Even though this document
does not specify the use of these additional keys, they may be used
for creating additional security within HTTP authentication or some
other security mechanism.
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RFC 3310 HTTP Digest Authentication Using AKA September 20025.6 Replay Protection
AKA allows sequence numbers to be tracked for each authentication,
with the SQN parameter. This allows authentications to be replay
protected even if the RAND parameter happened to be the same for two
authentication requests. More importantly, this offers additional
protection for the case where an attacker replays an old
authentication request sent by the network. The client will be able
to detect that the request is old, and refuse authentication. This
proves liveliness of the authentication request even in the case
where a MitM attacker tries to trick the client into providing an
authentication response, and then replaces parts of the message with
something else. In other words, a client challenged by Digest AKA is
not vulnerable for chosen plain text attacks. Finally, frequent
sequence number errors would reveal an attack where the tamper
resistant card has been cloned and is being used in multiple devices.
The downside of sequence number tracking is that servers must hold
more information for each user than just their long-term secret,
namely the current SQN value. However, this information is typically
not stored in the SIP nodes, but in dedicated authentication servers
instead.
5.7 Improvements to AKA Security
Even though AKA is perceived as a secure mechanism, Digest AKA is
able to improve it. More specifically, the AKA parameters carried
between the client and the server during authentication may be
protected along with other parts of the message by using Digest AKA.
This is not possible with plain AKA.
6. IANA Considerations
This document specifies an aka-version namespace in Section 3.1 which
requires a central coordinating body. The body responsible for this
coordination is the Internet Assigned Numbers Authority (IANA).
The default aka-version defined in this document is "AKAv1".
Following the policies outlined in [5], versions above 1 are
allocated as Expert Review.
Registrations with the IANA MUST include the version number being
registered, including the "AKAv" prefix. For example, a registration
for "AKAv2" would potentially be a valid one, whereas a registration
for "FOOv2" or "2" would not be valid. Further, the registration
MUST include contact information for the party responsible for the
registration.
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RFC 3310 HTTP Digest Authentication Using AKA September 2002
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